The invention relates to a process for fractionating a mixture which comprises one or more amines, water, low-boilers with or without high-boilers.
In the reaction of ammonia, primary or secondary amines with alcohols or with aldehydes in the presence of hydrogen, a reaction product produced, inter alia, is water, which frequently forms an azeotropic amine/water mixture with the product amine formed. In addition, the product mixture comprises low-boilers having a lower boiling point than that of the amine/water azeotrope, for example unreacted ammonia, or starting amine, and high-boilers having a higher boiling point than that of the product amine, for example higher-molecular-weight byproducts.
GB 1,102,370 describes a process for extractive distillation of an ethylenediamine/water mixture in which, in a first distillation column, the ethylene/water crude mixture is evaporated and the ascending vapor is brought into contact with aqueous sodium hydroxide solution flowing in countercurrent. At the top of the first column a low-water ethylenediamine/water mixture is obtained which has an amine concentration above the azeotropic point, which mixture is further distilled by simple rectification in a second column. At the top of the second column, pure ethylenediamine is obtained and at the bottom of the second column an ethylenediamine/water mixture is obtained which is combined with the crude mixture and recycled to the extractive distillation.
DE-A 29 02 302 describes a process for separating ethylarnine mixtures in which a diethylamine-, triethylamine-, ethanol-, water- and possibly monoethylamine-containing mixture is extracted with water and a water-immiscible solvent, an aqueous phase and a water-immiscible phase being obtained. The two phases are separated and further worked up by distillation. Water-immiscible solvents used are n-butane, n-hexane and cumene.
DE-A 27 23 474 describes a process for fractionating a water-, monoethylamine-, diethylamine- and triethylamine-containing mixture, in which an essentially anhydrous mixture of monoethylamine and diethylamine and triethylamine is separated off by distillation and, by distillation, monoethylamine is separated off from the anhydrous mixture of monoethylamine and diethylamine.
EP-A 0 881 211 describes a process for preparing anhydrous 2-amino-1-methoxypropane in which, in an extraction step, a 2-amino-1-methoxypropane-containing aqueous reaction mixture is admixed with sodium hydroxide solution, forming a sodium-hydroxide-containing aqueous phase and a 1-amino-1-methoxypropane-containing phase, the aqueous phase is separated off and, in a distillation step, the 2-amino-1-methoxypropane-containing phase is distilled, an azeotrope of water and 2-amino-1-methoxypropane first being produced, which is recycled to the extraction step, and then anhydrous 2-amino-1-methoxypropane then being produced.
It is an object of the present invention to provide an improved process for fractionating water-containing crude amine mixtures from amine synthesis, which process is suitable for a multiplicity of different crude amine mixtures.
We have found that this object is achieved by a process for fractionating an amine-containing mixture which comprises one or more amines, water, low-boilers and high-boilers, having the steps (i) to (iv):
(i) separating off by distillation low-boilers from the amine-containing mixture,
(ii) separating off by distillation high-boilers from the amine-containing mixture,
(iii) extracting the amine-containing mixture with sodium hydroxide solution, producing an aqueous, sodium-hydroxide-containing first phase and an aqueous-organic, amine-containing second phase,
(iv) distilling the aqueous-organic second phase, producing and an essentially anhydrous amine, an amine/water azeotrope and recycling the amine/water azeotrope to the extraction step (iii).
Surprisingly, separating off high-boilers before carrying out extraction step (iv) avoids the unwanted formation of solids in the extractor. The inventive process is preferably carried out continuously.
The inventive process can be carried out in such a manner that, in the distillation step (iv), the amine/water azeotrope is produced first and then the essentially anhydrous amine is produced. xe2x80x9cAminexe2x80x9d is also taken to mean a mixture of a plurality of amines.
In a preferred embodiment, in distillation step (iv), the amine/water azeotrope is produced as sidestream takeoff in the enrichment part of the distillation column and recycled to the extraction step (iii), and the essentially anhydrous amine is produced as sidestream takeoff in the stripping part of the distillation column, further low-boilers are produced as overhead takeoff and further high-boiler-containing amine is produced as bottom-phase takeoff.
The further high-boiler-containing amine produced as bottom-phase takeoff is preferably recycled to step (ii).
In a subsequent distillation step (v) the essentially anhydrous amine produced in step (iv) can be further fractionated.
In a further preferred embodiment, in distillation step (iv), the amine/water azeotrope is produced as sidestrearn takeoff in the enrichment part of the column and recycled to the extraction step (iii), and further low-boilers are produced as overhead takeoff and the essentially anhydrous amine is produced as bottom-phase takeoff.
In a subsequent distillation step (v), the essentially anhydrous amine produced in step (iv) can be further fractionated.
Removing a low-boiler fraction as overhead takeoff from the distillation column in step (iv) avoids accumulation of low-boilers due to recycling amine/water azeotrope to extraction step (iii) in the continuous procedure.
A downstream distillation step (v) is required to produce the pure amines, if the starting mixture comprises two or more amines which, with water, form azeotropes having very similar boiling points. Examples of this are azeotropes having boiling points which differ by no more than 10xc2x0 C.